1. Field of the Invention
The present invention relates to an apparatus and method for forming an aperture of a vertical cavity surface emitting laser (VCSEL) by selective oxidation and, more particularly, to an apparatus for and method of forming a precise aperture of a VCSEL, while monitoring resonance peak variations through an optical spectrum analyzer.
2. Description of the Related Art
Unlike edge emitting laser devices, VCSELs emit a substantially circular beam in the direction in which semiconductor material layers are stacked, so that it is unnecessary to adopt an optical system for shaping the output beam. Also, the size of VCSELs is reducible, and thus it is easy to integrate a plurality of VCSELs in a 2-dimensional array on a single semiconductor wafer. These advantages have expanded the application fields of VCSELs, for example, into optical communications, electronic calculators, audio and video apparatuses, laser printers, laser scanners and medical equipment.
In order to enhance power output characteristics by guiding the flow of current supplied through electrodes, a VCSEL is provided with a high-resistance region in an upper reflector. The high-resistance region can be formed by implanting protons or ions into the upper reflector, and by selectively oxidizing the peripheral region exclusive of an aperture that guides the flow of current, by adjusting the oxidizing time. A disadvantage of the implantation method lies in that it lacks repeatability in high volume manufacturing, because of a non-uniform distribution of protons or ions.
On the other hand, as for the selective oxidation method, after depositing an Al1xe2x88x92xGaxAs layer on a lower portion of an upper reflector, which is to be a high-resistance region, the resultant structure is etched, resulting in individual VCSELs on a wafer. Next, the VCSELs are left in an oxidation atmosphere for a predetermined period of time, to allow diffusion of vapor into the peripheral portion of the Al1xe2x88x92xGaxAs layer. As a result, an oxide insulating layer, Al2Ox layer, is formed at the peripheral portion as the high-resistance region, which limits flow of current, thereby resulting in an aperture surrounded by the high-resistance region.
The oxidative diffusion rate in forming an aperture of a VCSEL is highly sensitive to the temperature of a furnace for use in the oxidative diffusion, oxidation time and the amount of oxygen supplied into the furnace. A variation in the diffusion rate is a fatal problem in mass production that requires high repeatability, and in forming a particular size of the aperture.
To overcome these problems, it would be desirable to employ a furnace with precise temperature control, and to precisely adjust oxygen or vapor pressure in the furnace. However, in the case where the length of an oxidative diffusion region needs to be adjusted on the order of xc2x11xcexcm, precisely adjusting temperature and oxygen or vapor pressure is insufficient. In other words, the area of the Al1xe2x88x92xGaxAs layer to be oxidized during the process needs to be measured, and the length of the oxidative diffusion region must be monitored through a visual system. In practice, it is difficult to construct an optical system having a high enough magnification ratio to easily observe something so small.
The size of the aperture can be adjusted using a dummy sample. In particular, the length of the oxidative diffusion region is measured with respect to dummy samples so as to determine an appropriate oxidation time period. Then, the obtained oxidation time period is applied to form a desired aperture on an actual sample. However, the repeatability of this method is poor.
To solve the above problems, it is an objective of the present invention to provide an apparatus and method for forming an aperture of a vertical cavity surface emitting laser (VCSEL) by selective oxidation, in which the degree of oxidation during the formation of the aperture is adjusted by measuring a variation in a resonance peak, which depends on the size of the aperture, using an optical spectrum analyzer, so that the size of the aperture can be precisely adjusted.
According to an aspect of the present invention, there is provided an apparatus for forming an aperture of a vertical cavity surface emitting laser (VCSEL) by selective oxidation, comprising: a furnace having a first window and a second window for transmitting light, and a stage for supporting a wafer for the VCSEL with a pre-oxide layer where an aperture is to be formed; a light source placed outside the furnace, for emitting light through the first window onto the top of the wafer for the VCSEL seated on the stage; and an optical spectrum analyzer for detecting the light intensity by receiving light reflecting from the top of the wafer for the VCSEL and passing through the second window, wherein the size of the aperture created can be controlled based on the variation of a peak wavelength, which varies according to the degree of oxidation of the pre-oxide layer.
According to another aspect of the present invention, there is provided a method of forming an aperture of a VCSEL by selective oxidation, comprising: providing a furnace having a stage, and first and second windows for transmitting light, and placing a wafer for the VCSEL with a pre-oxide layer where an aperture is to be formed, on the stage; emitting light onto the top of the wafer for the VCSEL seated on the stage through the first window; receiving light reflecting from the wafer for the VCSEL and passing through the second window; supplying at least one of oxygen and water vapor into the furnace to create an oxidation atmosphere; detecting a peak wavelength variation from light received through the second window, which varies according to the degree of oxidation of the pre-oxide layer; and adjusting power and/or oxidation atmosphere of the furnace based on the detected peak wavelength variation to cease the oxidation reaction, thereby adjusting the size of the aperture.